Granular and prilled urea are the most widely used and agriculturally important nitrogen fertilizers. Urea as a form of nitrogen is barely taken up, or not at all, as it is rapidly hydrolyzed in the soil by the enzyme urease. Urease is ubiquitious in soil bacteria and fungi and it converts the urea back into ammonia and carbon dioxide (Mobley et al. Microbiol. Rev. 1995, 59, 452-480). During this process, gaseous ammonia may be released into the atmosphere prematurely and is then no longer available in the soil for fertilizing plants, thus reducing the efficacy of the fertilization.
One approach toward improving the availability of the nitrogen to the root system of plants over an extended period of time has involved the use of a nitrogen stabilizer such as a urease inhibitor or a nitrification inhibitor. Urease inhibitors are compounds capable of temporarily inhibiting the catalytic activity of the urease enzyme on urea in moist soil (Gardner, Ag Retailer, November 1995; Marking, Soybean Digest, November 1995, Varel et al., Journal of Animal Science 1999, 77(5); Trenkel “Slow and Controlled-Release and Stabilized Fertilizers, 2010). Slowing the urease-catalyzed transformation of urea to ammonium minimizes ammonia losses and allows time for absorption or dissipation of the nitrogen (N) forms into the soil. Reductions in ammonia volatilization from urease can range from 55 to over 99% (Watson et al., Soil Biology & Biochemistry 26 (9), 1165-1171, 1994), with a typical volatilization reduction of 75 to 80% in the field environment. One commercially used urease inhibitor is the compound NBPT, N-(n-butyl)thiophosphoric triamide, which is a pro-compound of its active oxygenated derivative, N-(n-butyl)phosphoric triamide (Phongpan et al., Fertilizer Research 41(1), 59-66, 1995). NBPT has been used as a coating on granular urea (see e.g. U.S. Pat. No. 5,698,003) or an additive to aqueous solutions of urea (see e.g. U.S. Pat. No. 5,364,438).
Nitrification inhibitors are compounds which inhibit the conversion of ammonium to nitrate and thus, also reduce nitrogen losses in the soil. Examples of nitrification inhibitors include, but are not limited to, dicyandiamide (DCD), 2-chloro-6-trichloromethylpyridine (nitrapyrin), 3,4-dimethylpyrazole phosphate (DMPP), 3-methylpyrazole (MP); 1-H-1,2,4-triazole (TZ); 3-methylpyrazole-1-carboxamide (CMP); 4-amino-1,2,4-triazole (AT, ATC); 3-amino-1,2,4-triazole; 2-cyanimino-4-hydroxy-6-methylpyrimidine (CP); 2-ethylpyridine; ammonium thiosulfate (ATS); sodium thiosulfate (ST); thiophosphoryl triamide; thiourea (TU); guanylthiourea (GTU); ammonium polycarboxilate; ethylene urea; hydroquinone; phenylacetylene; phenylphosphoro diamidate; neemcake; calcium carbide; 5-ethoxy-3-trichloromethyl-1,2,4-thiadiazol (etridiazol; terraole); 2-amino-4-chloro-6-methylpyrimidine (AM); 1-mercapto-1,2,4-triazole (MT); 2-mercaptobenzothiazole (MBT); 2-sulfanilamidothiazole (ST); 5-amino-1,2,4-thiadiazole; 2,4-diamino-6-trichloromethyl-s-triazine (CL-1580); N-2,5-dichlorophenyl succinanilic acid (DCS); nitroaniline, and chloroaniline.
While granular urea has been coated with NBPT and/or DCD to help prevent nitrogen loss, the disadvantages with coating granular urea is that either 1) a hygroscopic liquid carrier is used for the inhibitors, or 2) a solid carrier is used for the inhibitors which can result in residual dust which causes handling problems. These problems can be solved by incorporating the urease and/or nitrification inhibitor directly into the molten urea before it is granulated.
Solid DCD has been directly added to re-melted granular urea containing about 4 to 6 weight % water at 275° F. and subsequently passed through 1) an evaporator and 2) a granulator to form a granulated a homogeneous granular fertilizer containing about 1 weight % DCD (see U.S. Pat. No. 5,352,265). However, the high moisture content of this urea makes this product less desirable.
Similarly, a urea granule containing 0.2 weight % NBPT was produced by pumping a 56 weight % solution of 80% pure NBPT containing other impurities in N-methylpyrrolidone (NMP) at ca. 136 lbs/hour into a stream of re-melted urea at 275° F. for about 20 seconds. The molten urea-NBPT composition was subsequently granulated to form a homogeneous granular fertilizer (see U.S. Pat. No. 5,352,265). Granular urea containing 0.01, 0.025, 0.0375, 0.05, 0.075 and 0.1 weight % NBPT has also been produced by mixing a dilute 20 weight % solution of NBPT in 10 weight % NMP and 70 weight % propylene glycol for 1 to 15 minutes (see also Watson et al. (Soil Use and Management, September 2008, 24:246-253) (see H. Cantarella, “Evaluation of the urease inhibitor NBPT N-(n-butyl)-thiophosphoric triamide on the efficiency of urea fertilizer under Brazilian Soil conditions”, October 2003). However both of these compositions did not contain any nitrification inhibitor.
In addition, NBPT is costly to make and susceptible to decomposition during storage or upon heating, especially in a hygroscopic environment, like molten urea. Accordingly, there is a need to minimize the degradation of NBPT by reducing the water and impurity content in the composition, as well as the amount of NBPT used.
Further, an inherent problem with forming solid urea is that the urea is heated to or near its melting (crystalline phase change) point with a consequent increase in the biuret content. It is well known that biuret, formed by the condensation of two molecules of urea with the loss of one molecule of ammonia, is noxious to plant life since it exhibits a very active phytotoxic action. In addition, there are safety concerns because of possible exposure to ruminant animals. Biuret quickly forms ammonia at concentrations in the rumen fluid which can be toxic to the ruminant animal. While it is generally desirable that the urea have a maximum biuret content of 0.25% by weight, more preferably less, the increased times associated with mixing additional materials into molten urea results in higher biuret content.
Another problem with these prior methods is that both DCD and NBPT can be difficult materials to handle and costly. DCD has poor solubility in most solvents. Similarly, industrial grade NBPT is a waxy, sticky, heat-sensitive and water-sensitive material (see also WO 2010/045895 and U.S. Pat. No. 8,513,460). Because of the solubility issues of industrial grade NBPT and the temperatures involved in the injecting NBPT into molten urea (i.e. 275° F.), NMP has always been used as a co-solvent in the direct incorporation of NBPT into molten urea (see above, and Kincheloe, The manufacture, agronomics and marketing of AGROTAIN®. IFA Agro-Economics Committee Conference: ‘Plant Nutrition in 2000’, Tours, International Fertilizer Industry Association, Paris, France 1997). However, the agricultural use of NMP has environmental and regulatory constraints. While this solvent is ideal for incorporation process into molten urea because of its high boiling point and polarity, it is also difficult to remove from the final products, especially on the large scales required for efficient production of fertilizer compositions. Therefore, the ability to use less NMP is desirable.
Because most urea is produced in existing urea plants and the urea produced commercially does not contain a urease or nitrification inhibitor, the addition of NBPT or DCD has been done by re-melting granular urea. Accordingly, systems and apparati to perform these types of operations are not commercially available in urea production facilities and must be adaptable to existing urea manufacturing plants.
Accordingly, there is a need for improved compositions where a nitrogen stabilizer is combined with molten urea that uses substantially less NMP and/or nitrogen stabilizer, and contains less water, biuret and other impurities, but that still provides effective fertilizer granules. Further, there is a need for improved compositions that use less nitrogen stabilizer by minimizing degradation and other side-products formed during the process to make the composition. There is also a need for improved methods, apparati and systems for making and using the same. The above mentioned disadvantages can be solved by compositions, methods, apparati and systems according to the present invention.